The present invention relates to treated particles suitable for use in electrorheological fluids.
Electrorheological ("ER") fluids are fluids which can rapidly and reversibly vary their apparent viscosity in the presence of an applied electric field. ER fluids are generally dispersions of finely divided solids in hydrophobic, electrically non-conducting oils. They have the ability to change their flow characteristics, even to the point of becoming solid, when subjected to a sufficiently strong electrical field. When the field is removed, the fluids revert to their normal liquid state. ER fluids may be used in applications in which it is desired to control the transmission of forces by low electric power levels, for example, in clutches, hydraulic valves, shock absorbers, vibrators, or systems used for positioning and holding work pieces in position.
The prior art teaches the treatment of fine particles with surface coatings of various types in order to impart some particularly desired property to the particles. For example, U.S. Pat. No. 4,990,279, Ahmed, Feb. 5, 1991, discloses electrorheological fluids prepared from monomers which are polymerized in a low conductivity medium. The polymer particles are further modified through polymerization of a hydrophilic monomer to form a hydrophilic shell or globule around the particles, which constitutes a minor part of the particle.
U.S. Pat. No. 3,989,872, Ball, Nov. 2, 1976, discloses fine powders comprising yttira stabilized zirconia powders encased in a thin calcia shell, for plasma spray coating processes. The coating is accomplished by first forming a deposit of calcium carbonate on the individual particles and converting the calcium carbonate to calcium oxide by heating.
European publication 394,049, Oct. 24, 1990, discloses electrorheological fluids comprising a dispersed particulate phase which includes a plurality of composite particulate bodies, each having a core with an electrically conductive surface coated with a layer of electrically relatively non-conductive material, with the composite particulate body having a density substantially the same as the density of the carrier liquid.
PCT publication WO90/00583, Jan. 25, 1990, discloses an electroviscous fluid comprising electrically polarizable aggregate particles dispersed in a dielectric fluid. A substantial portion of the aggregate particles comprise a core and an electrically insulative shield. The shield can be e.g. a resin, a plastic foam, or a ceramic glaze.
Japanese publication 63-97694, Apr. 28, 1988, discloses an electroviscous fluid containing particles with a three-phase structure which comprises a core of an organic solid particle, a conductive thin-film layer formed on its surface, and an electric insulating thin-film layer formed thereon. Examples of insulating materials include polyvinyl chloride, polyamide, polyacrylonitrile, polyvinylidene fluoride, wax, asphalt, varnish, silica, alumina, rutile, barium titanate, and the like.
Japanese publication 64-6093, Jan. 10, 1989, discloses an electroviscous fluid comprising an oily medium and dielectric fine particles consisting of a conductive particle coated with an electric insulating film having 1 .mu.m or less thickness, and containing no water substantially. Representatives of the insulating materials include organic synthetic polymers, organic natural polymers, inorganic compounds such as silica, alumina, aluminum hydroxide, barium titanate and the like.
Japanese publication 3-93898, Apr. 18, 1991, discloses an electroviscous fluid consisting of fine particles which have a conductive layer on their insulating surface, which layer is coated further with an insulating film. Materials for the outermost film include silica, titania, alumina, tantalum, and styrene and epoxy resins.
U.S. Pat. No. 4,937,060, Kathirgamanathan, Jun. 26, 1990, discloses an inorganic powdery or granular material which is coated with a coating of an inherently conductive polymeric material. Conductive calcium carbonate and conductive aluminum trihydrate, when compounded into a polymer such as poly(vinyl chloride), are taught to e.g. impart conductivity to the polymer.